WASHINGTON (CNN) -- The capture of two cocaine-laden semi-submarines in the past week has highlighted the increasing use of the vessels, which now transport one-third of illicit drugs in the eastern Pacific, a major route between Colombia and the United States, U.S. authorities said Friday.

This 60-foot semi-submersible vessel sank after U.S. authorities boarded it September 17.

The first seizure occurred September 12, when a Coast Guard team, under cover of darkness, boarded a 59-foot semi-submersible 370 nautical miles southwest of Guatemala.

The startled Colombian smugglers reversed the vessel in an effort to throw the boarding party overboard, but the Coast Guard was able to seize the craft, its four-person crew and its 237 bales of cocaine.

Just five days later, on Wednesday morning, a 60-foot semi-submersible was seized about 200 nautical miles south of Guatemala. As the boarding team unloaded the last few bales, the Coast Guard said, the unstable vessel began to take on water through its exhaust vents and sank. Watch what was found inside one of the subs »

U.S. Navy, Coast Guard and Drug Enforcement Administration officials say South American drug cartels are turning to semi-submersible vessels, which have a low profile to avoid detection, because of the government's success at thwarting other smuggling techniques, including the use of fishing trawlers and speed boats.

Between 2000 and 2007, U.S. authorities said, there were 23 drug sub "events" that they know of. But the number of sightings has mushroomed. There have been 62 events in the first three quarters of fiscal 2008, officials said.

"This is a serious threat that is growing, both in volume of cocaine shipped and in sophistication of the craft," said Rear Adm. Joseph R. Castillo, the Coast Guard's director of response policy.

The Coast Guard has captured only a small number of the vessels known or believed to have crossed the area, it said. In some cases, the crews have scuttled the semi-subs to avoid prosecution.

Coast Guard officials say the semi-subs typically are built in the jungles of Colombia, are 25 to 65 feet long, have crews of four or five people and are capable of speeds of up to 13 knots with a range of over 5,000 nautical miles.

The vessels' low profile makes them extraordinarily hard to detect, either visually or by radar. Intelligence played a role in the two most recent busts, officials said.

Locating semi-subs "is more difficult than finding a needle in a haystack," said Michael Braun, chief of operations for the DEA. "When you look at the vast amount of sea that they have to patrol to find these things, we're talking about an area, I believe, that's twice the size of the United States."

U.S. Drug Czar John Walters said Friday that the success in capturing subs has contributed to a reduced supply of cocaine on U.S. streets, evidenced by high cocaine prices and declining quality.

"Today we are now over 20 months into significant and sustained shortages of cocaine on the streets of the United States," Walters said. "Last year alone, workplace drug testing [that returned positive] for cocaine dropped 19 percent. This is closing the vise. And in the process of closing the vise, [smugglers] have tried new techniques."

Walters said smugglers are reluctant to sail on the semi-subs because they are dangerous.

"Some of them are little better than death traps, which is the degree to which they've been pressed to try to continue this deadly trade, even though they can't keep up."

The Coast Guard and the drug czar support legislation to impose stiff penalties for operating a semi-sub on the open seas.

"We want to criminalize the use of these in the high seas so that it's not OK to just scuttle them, get rid of the drugs and be free from prosecution," Walters said.